Method for processing ash, particularly fly ash

ABSTRACT

Disclosed is a method for processing ash, particularly fly ash, in which method several elements are separated from the ash. In the method both noble metals and rare earth elements are separated.

The present invention relates to a method for processing ash, particularly fly ash, in which method several elements are separated from the ash.

The EU is increasingly dependent on the import of not only primary energy sources, but also of industrial raw materials. The EU is therefore more exposed and vulnerable than other states to the effects of market distortion. Some of these industrial primary raw materials are used in the manufacture of so-called high-technology products. The products in question are utilized in, among others, environmental-technology solutions, to promote the improvement of energy efficiency and the reduction of greenhouse-gas emissions.

In 2010, the European Commission analysed the economic importance and availability risk of a total of forty-one raw materials used by industry. Fourteen of the minerals and metals analysed were deemed critical to the industrial activity of the European Union, because they have a significant economic effect on key sectors, or their availability and replacement contain significant risks. The raw materials classified as critical are antimony, indium, beryllium, magnesium, cobalt, niobium, calcium fluoride, the metals of the platinum group, gallium, the rare earth elements (lanthanums), germanium, tantalum, graphite, and wolfram.

Every year, about one million tonnes of waste are created in Finnish power plants, mostly ash arising from combustion and sulphur removal. The ash is either so-called bottom ash, or fine-particle fly ash collected from flue-gas filters. The ash typically contains mainly incombustible minerals, silicates, and possibly also heavy metals. Most of this ash, about 60%, is used as various earthworks, for example, in field structures and as a filler in landfill structures, as well as a batching material in concrete and cement, for example, as a raw material in cement and in building boards. These exploitable ashes are typically utilized as such and in the state in which they left the power plant. Most, about 55%, of these exploitable ash wastes arise in coal burning.

The low degree of utilization has partly been due to relatively cheap final disposal costs and the statutory waste status of ash, as well as tight limit values for substance contents, for example, in fertilizer and earthwork use. Changing tax procedures and steadily rising transport costs place a continually increasing cost pressure on power plants, in terms of ash treatment.

In waste exploitation, the point of departure is to meet statutory obligations. Attempts have been made to use legislation to facilitate the use in earthworks of bottom and fly ash from the combustion of coal, peat, and wood-based material. However, the quality of the ashes must be defined and monitored. By also limiting the thickness of a final-disposal structure, the aim has been to prevent the creation of uncontrolled sorting areas. For example, fly ash will consolidate, if water is added to it and it is compacted. Fly ash can then be used, for example, as structural layer in a road.

Most of the ash in mixed combustion arises in fluid-bed-combustion power plants. The quality of wood ash also varies between the different parts of a tree. For example, the metals contents relative to energy content are greater in the bark and branches than in the trunk. The element contents of the ground also vary according to time and place, which affects the quality of the ash. When they grow, trees and plants absorb minerals and elements along with water from the ground, which during growth enrich the structures of trees and plants. Indeed, it can be assumed that plants manifest the geology of the area in which they grow, and that the variation in the element contents of the ground can also be detected in the composition of the ash.

Quite a large number of solubility studies exist of the fly ash of coal, in which the emphasis is generally on the solubility of specific harmful substances. The solubility of other metals from the fly ash of coal has been shown to be quite small. The solubility properties of the ashes from mixed combustion generally correspond typically to the solubility of ash formed from the combustion of coal and peat.

The share of biofuels in energy production is increasing, due to the aims and objectives of climate and energy policies. The most important effects on the increase in the use of are the EU's until year 2020 statutory greenhouse-gas reduction goals and the aim of increasing renewable energy. The reduction goal for greenhouse gases is 20% of the level of 1990 and the goal for increasing renewable energy is 20% of total energy consumption compared to the level of 2005. The increasing use of biofuels in power plants changes not only the combustion event but also the composition of the ash that is created.

There are several methods, most of which have been developed to make the processing of ashes suitable for landfill. Dry ash can be air-classified, in which the ash is divided into various fractions on the basis of particle size and specific weight. Relatively most soluble substances and heavy metals exist in small particles, which can be separated by air-classification. Correspondingly, soluble substances can be separated using water or acid washing. However, washing leads to costs and creates waste water. The solubility properties of ash can also be affected by storage. When it ages, ash reacts with air, when its solubility changes. Heavy metals can be removed by thermic methods. Heating procedures consume much energy and do not completely purify the ash.

Finnish patent number 101572 discloses a method, which seeks to stabilize fine ash into larger ash particles. However, the method in question requires a combustion plant of a specific type. In addition, the method is unsuitable for processing fly ash, which is removed only in the final stage of the combustion process. The use of fly ash for earthworks is problematic due to its capillary structure. In practice, a layer formed of fly ash is susceptible to frost heave even when compacted.

Japanese patent application number 2007321239 discloses the recovery of copper from fly ash. In the method, the fly ash is treated with additives and the mixture is processed at a high temperature. The method is suitable for only a limited number of elements and requires a great deal of energy while giving only a modest yield.

The invention is intended to create a new type of method for processing ash, particularly fly ash, which is more efficient than previously and by means of which a greater number of more valuable elements than previously can be isolated from the ash, so that the costs arising from the ash can be substantially reduced. The elements separated can be re-utilized, for example, as raw materials in industrial processes. The characteristics features of the present invention are stated in the accompanying Claims. In the method according to the invention, ash is processed in stages, so that the numerous elements are recovered in a controlled manner. In addition, the substances used in the processes are cheap and sage and can be recycled or otherwise exploited after the process. The isolating processes can be linked in a chain, thus making the total process efficient, which increases the yield of elements. At the same time, the purity of the elements is good and the residue of the isolating processes can be utilized as a raw material, instead of being waste as previously. In this way, the processing of ash, which previously mostly only gave rise to costs, becomes a profitable business activity.

In the following, the invention is described in detail with reference to the accompanying drawings depicting some embodiments of the invention, in which

FIG. 1 shows the according to the invention schematically,

FIG. 2 a shows the first partial stage of the method according to the invention,

FIG. 2 b shows the second partial stage of the method according to the invention.

FIG. 1 shows the method according to the invention stage by stage. The method is intended for the processing of ash, particularly fly ash. In the method, several elements are separated from the ash. In the method according to the invention, both noble metals and rare earth elements are separated, of which there are, surprisingly, significant amounts in ash and particularly in fly ash. Thus, even the processing of fly ash is profitable and, at the same time, the processed ash can be utilized more widely than previously. In other words, instead of the previously harmful elements, by means of the method according to the invention economically significant elements can be separated from the ash.

Ash is known to be poorly soluble. Therefore, in the invention it has been resulted to use staged processing, which is, however, preferably continuous. Part of the processing can also operate on the batching principle, allowing the process to proceed in specific cycles while being nevertheless continuous. In the invention, the elements are isolated in a two-stage extraction process 10 and 11. In other words, there are two extraction processes one after the other. The isolation of the elements can thus be standardized and the desired elements obtained from the extraction processes can be isolated. In the first extraction process 10, noble metals are isolated and in the second extraction process 11 rare earth elements are isolated. Both of the extraction processes can be optimized separately, thus increasing the yield of elements.

Generally, in the extraction solids are dissolved in a liquid, such as water. In dissolution, it is sought to made the substances contained in a solid dissolve as completely as possible. However, it has proven to be challenging to dissolve ash, so that in the first extraction process 10 according to the invention the noble metals are dissolved using specifically a solution of oxalate in water 12, in which case the elements are made to dissolve selectively. It was observed during the development of the method that an acid solution with an oxalate content effectively dissolves noble metals, without, however, dissolving rare earth elements. The water solution with an oxalate content is formed using either oxalic acid (H₂C₂0₄) or ammonium oxalate ((NH₄)₂C₂0₄). In addition, the extraction solution should be acidic. The greatest efficiency of the extraction solution is obtained when the pH of the solution is adjusted to a value 2 or less. The extraction typically lasts from hours to tens of hours, depending on the properties and concentration of the solution. The oxalate extraction solution 21 obtained from the first extraction process 10 is led to a first step 13, which will be depicted in detail later, in order to isolate the noble metals.

During the development of the method, it was observed that undissolved ash 14 remained in the first extraction process 10. Because the oxalate-content water solution 12 had not dissolved all the solids, another substance promoting dissolving was selected. In the second extraction process 11 according to the invention, rare earth elements are dissolved from the ash that did not dissolve in the first extraction process 10 by using a solution 15, which is a mixture of sulphuric and nitric acids. Sulphuric acid was chosen as the extraction solution for this stage because in is not an acid that causes particularly much corrosion and is thus suitable for an industrial process. In addition to this the sulphuric acid is obtained as a by-product from different industrial processes and it is thus reasonable cheap mineral acid. During the development of the method, it was observed that the extraction efficiency of sulphuric acid increases if nitric acid is added to it. The mixture in question was observed to be extremely effective and a large amount of rare earth elements were dissolved. In other words, washed ash that has not dissolved in the previous stage is extracted in the second stage using a mixture of sulphuric and nitric acids. The extraction typically lasts from hours to tens of hours, depending on the properties and concentration of the solution. The extraction solution 30 containing sulphuric and nitric acid obtained from the second extraction process 11 is then led to a second step 16, which will be depicted in greater detail later, in order to isolate rare earth elements.

The solutions created in the extraction processes 10 and 11 are thus precipitated in two steps. In the first step 13, noble metals are precipitated and in the second step 16 rare earth elements are precipitated. The extraction processes and the steps can be separate, but the extraction processes and steps are preferably linked to each other and arranged to operate seamlessly. Thus, the total process and equipment become compact. At the same time, it becomes possible to recycle the substances used in the processes and the yield of elements is maximized. In addition, energy consumption is reduced, as heat recovery can be exploited in the equipment.

Undissolved ash 17 is still left over from the two consecutive extraction processes, but this is, however, mainly a residue 18 containing silicate. In the residue, there can be small amounts of elements, which can, if required, be isolated using one or more additional extraction processes (not shown). However, already after two extraction processes a significant proportion of the elements will have been isolated. At the same time, the harmful substance will also have been removed, in which case the silicate-content residue can be exploited more extensively than previously, without waste status. The undissolved residue contains mainly silicates and can be exploited, for example, in earthworks, such as in the bottom layers of roads, as well as in cement manufacture.

According to FIG. 1, the ash 14 and 17 that was undissolved in the extraction processes 10 and 11 is washed with water in washing stages 19 and 20, before the next treatment. In other words, the extract is separated from the undissolved ash, which is washed with water. In this way, the dissolved elements and the extraction solution are recovered. At the same time, residues of the extraction solution, which could be detrimental to the following process or the exploitation of the residue, do not remain in the insoluble ash. In addition, the wash solution formed in the washing is returned to the extraction process 10 or 11 after the washing stage 19 and 20. Thus, even the wash water and the elements it contains are brought into the steps, in this example steps 13 and 16. In the washing, possible impurities are also removed, which are led to further treatment along with the insoluble residue.

FIG. 2 a shows the first step 13 of the method according to the invention, in which the oxalate solution 21 obtained from the first extraction process 10 is processed in at least two stages. First, sulphide and a first precipitation solution 22 containing ammonium chloride are added to the oxalate solution 21, in order to separate iridium and copper as a precipitation process 24. Sodium sulphide (Na₂S), or some other chemical with a sulphide content, as well as ammonium chloride (NH₄Cl) is used as the first precipitation solution 22. The noble metals are precipitated mainly as sulphides, so that sodium sulphide is one of the cheapest sulphide-content reagents. During the development of the method, it was observed that the addition of ammonium and chloride ions improved the precipitation of gold from the extraction solution. The contents of sodium sulphide (Na2S) and ammonium chloride in the solution used for precipitation should be 0.6±0.1 mol/l. and 2.5±0.2 mol/l. The solution is heated and allowed to cool, when a precipitate 23 is formed. In this first precipitation process 24, the pH of the oxalate solution 21 is arranged to be 1.5 using an adjusting solution 25, when the aforementioned elements will be isolated precisely. The adjusting solution 25 is preferably hydrochloric acid (HCl) or NH₃. The pH of the solution 26 remaining from the first precipitation process 24 is adjusted in the second precipitation process 27 in order to precipitate the remaining noble metals. In this second precipitation process 27, the pH of the solution 26 is arranged to be 8.5, when the remaining valuable elements will be precipitated. In this case too, the adjusting solution 28 is NH₃. After the pH has been raised, the solution is heated then allowed to cool and the precipitate is isolated. The precipitate 29 contains gold and platinum metals, also well as additionally iron and aluminium. The solution separated from the precipitate contains rubidium and magnesium. The various noble metals obtained from the precipitates 23 and 29 from the precipitation processes 24 and 27 are separated using some known technique. One possible way is to dissolve the precipitate using mineral acids, after which the noble metals can be isolated electrolytically.

FIG. 2 b shows the second step 16 of the method according to the invention, in which an oxalic-acid solution 31 is added to the extraction solution 30 obtained from the second extraction process 11, in order to separate rare earth elements as a third precipitation process 32. Surprisingly, the oxalic-acid treatment now precipitates the rare earth elements. Oxalic acid is used, because, according to the chemical properties of rare earth elements, they precipitate from an acid solution as oxalates. In addition, in this third precipitation process 32 the pH of the extraction solution 30 is arranged to be 1.5±0.3 using an adjusting solution 33. In this way the most efficient precipitation is achieved. If the pH is raised higher than this, other metals contained in the extraction solution will begin to accumulate as impurities in the precipitate. After the addition of oxalic acid and the adjustment of the pH, the solution is heated and allowed to cool, when a precipitate will form. The precipitate formed is separate from the solution. The precipitate containing rare earth elements mainly as oxalates is washed with water and the wash water is combined with the previously separated solution. The various rare earth elements from the precipitate 34 obtained form the third extraction process 32 are separated using some known technique. The precipitate can be heated, for example at a temperature of 800 degrees, when oxides of the rare earth elements will be formed. The exploitable product will then be a mineral concentrate containing oxides of rare earth elements.

The various stages of the separation process and the extraction and precipitation substances and additives together with their contents are described above. The extraction processes 10, 11 and/or the precipitation processes 24, 27, 32 are boosted by adjusting the temperature, adjusting the pressure, agitating the solution, treating the solution mechanically, and/or directing ultrasound to the solution. Particularly a sufficiently high temperature and agitateing, combined with ultrasound will promote and accelerate especially the extraction processes. In tests, particularly by using ultrasound the elements were made to dissolve almost completely.

In the separation process, noble metals and rare earth elements are recovered from ash. In addition, in the second stage 27 of the precipitation process the pH is adjusted by using ammonia and after the second stage 27 the solution is treated in such a way that the remaining nitrogen can be used as a fertilizer. This allows the nitrogen to be exploited. A second example of a preferred total process is the recycling of oxalate. According to the invention, the oxalic acid used in the rare-earth-elements precipitation process 32 is recycled to the first extraction process 10. This reduces material costs and for its part permits the creation of a continuous process.

Thus, according to the invention ash, particularly fly ash, is subjected to extraction in two stages. The extract arising in the first extraction process contains metals, such as copper and especially noble metals. Noble metals are ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold. Of these, ruthenium, rhodium, palladium, osmium, iridium, and platinum are considered to be platinum-group metals. For its part, the extract arising in the second extraction process contains rare earth elements. Rare earth elements are scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The elements in question are precipitated from both extracts in separate steps.

By means of the method according to the invention, it is possible to process ash, particularly fly ash, arising in the combustion of solid fuels in energy production. In the processing, valuable noble metals and rare earth elements are effectively isolated. In the isolation, extraction and precipitation processes are used, which are linked to each other to form a continuously operating totality. The method is preferably a continuously operating process, in which the ash is treated to form a solid concentrate containing desired and valuable elements. By means of the method, most of the ash is processed into an exploitable form and, at the same time, economically valuable elements are recovered.

The extract of the first extraction process, containing valuable elements, is processed on the batch principle in several consecutive stages, in order to bring the elements into a solid form. A solution of the correct strength containing sodium sulphide or other sulphides is added in a controlled manner to the solution of the first extraction process, in order to precipitate iridium and copper. After this, the pH of the remaining solution is raised by means of a solution of ammonia in water, in order to precipitate noble metals. Oxalic acid of the correct concentration is added in a controlled manner to the mixture arising in the second extraction process, in order to bring rare earth elements into a solid form. In each precipitation stage, the solution being treated is allowed to react with the reagent for sufficiently long period of time for the maximum yield to be obtained. The desired elements remain as ions in the extraction solution along with the other soluble elements. The precipitates of the precipitation stages can contain undesirable elements, which are separated from the desired elements in actual metallurgical processes. In the method according to the invention, the extraction processes are optimized separately, so that the noble metals and rare earth elements are in their own extraction solutions. In other words, the extraction solutions are separated into solution fractions, in which the elements are concentrated evenly. Thus, the extraction processes have been advantageously kept in two extraction processes. In addition, the contents of undesirable elements in the solid precipitate created remain minimal. In a two-stage extraction process, two concentrates arise, a noble-metal concentrate and a rare-earth-element concentrate, which are processed separately.

There is a great deal of use for the method, as the use of biomasses and waste in energy production is being increased greatly. Ash formed in the combustion of coal too can be processed using the method, through higher concentrations of desired elements are in biomasses, such as tree stumps. However, in coal ash there is much palladium, gold, and iridium. When they grow, trees and other plants absorb minerals and elements from the ground along with water, which are concentrated in the structures of the trees and plants during growth. The benefit obtained by means of the method increases the recovery of valuable elements. At the same time, the load on the environment is significantly reduced. By exploiting ash, blasting and other mining operations are avoided. In addition, the reagents used in the extraction of ash are considerably more environmentally-friendly than the reagents used in the extraction of mineral substances. At the same time, the amount of ash to be finally disposed on is reduced.

In addition to noble metals, rare earth elements can be isolated using the method. Rare earth elements appear in very small concentrations in groundwater, from where they accumulate in, for example, trees. In research, it has been surprisingly observed that tree stumps in particular contain high concentration of rare earth elements. Peat too contains rare earth elements, the concentrations depending on the geology of the area. The discovery of noble metals increases very steeply the value of the concentrate that can be obtained, as their price level has remained nearly unchanged at a high level.

In tests, the total yield percentages vary, according to the quality of the ash in terms of desired elements, in the range 80-90%. Two-stage extraction has proven advantageous, as in the first extraction process most of the noble metals as well as rubidium and gallium dissolve in a 0.75-M ammonium oxalate solution. When testing the extraction process, it was observed that a good yield was obtained by using heating and ultrasound. In addition, standing the solution between short ultrasound treatments boosted the yield. In the second extraction stage, the rare earth elements and some of the noble metals dissolved in a mixture of sulphuric and nitric acid, in which the sulphuric-acid content is 0.3-1.0 mol/l and the nitric-acid content 0.05-0.25 mol/l. The following can be stated concerning one optimized example of the extraction processes: 10 ml 0.75-M ((NH₄)₂C₂O₄)) was added to 500 mg ash and the solution was treated using ultrasound. The extract was then separated and the residue was transferred to a second extraction process, in which 10 ml 0.45-M H₂SO₄+5 ml 0.2-M HNO₃ was added. The solution was treated with ultrasound and the extract filtered. The residue remaining from the extraction processes contained mainly undissolved silicates. The extraction processes were strong, so that the solubility of the residue is very low. Thus the residue can be utilized in, for example, earthworks or concrete manufacture.

The rare earth elements are precipitated from the extraction solution for example as follows: 1 ml oxalic acid is added to the 20 ml of extraction solution of the second extraction process and the pH is adjusted to the value 1.5 using NH₃, while constantly agitating. The solution is heated in a 65° C. water bath for 40 minutes. The solution is centrifuged and the solution phase is separated and diluted with water. The precipitate is allowed to dry, after which the precipitate is dissolved with the aid of ultrasound in 2 ml aqua regia and diluted with water to a volume of 10 ml. The element concentrations are measured using, for example, an inductively coupled plasma-optical emission spectrometer (ICP-OES). Using oxalic-acid precipi-tation, about 80% of the rare earth elements are precipitated. The best amount of oxalic acid is about tens times the mass of rare earth elements. The consumption of oxalic acid is mainly affected by the elemental composition of the ash. The processing of the fly ash analysed in the tests would consume about a kilogram of oxalic acid to each tonne of ash. The adjustment of the pH would correspondingly consume about 2500 litres of ammonia 5 mol/L water solution. The consumption of other reagents would be about 2500 litres of 0.06 M Na₂S solution, about 2500 litres of 2.5-M ammonium-chloride solution, and about 2500 litres of sulphuric acid.

Fly-ash can also be processed as follows. A 200 ml 0.5-mol/l-oxalic-acid solution is added to a 10-gram ash sample. The ash sample is agitated mechanically for 2 h. The use of heating and ultrasound during agitation boosts the dissolving of the elements. After the first extraction stage, the ash can be separated from the solution, for example, by sedimentation. After this, the noble metals are precipitated from the solution as sulphides. A 300 ml 0.5-mol/l-sulphuric-acid solution is added to the residual ash and the mixture formed is agitated for 1 h. Stronger sulphuric acid than this can also be used in extraction, if the solution volume is reduced. The reduction of the solution volume also reduces the volume of the entire process, thus also reducing the process costs. In this case too, the use of heating and ultrasound boosts the solubility of the elements. After the second extraction stage, the residual ash contains mostly silicates. In addition, rare earth elements are precipitated as oxalates from the sulphuric-acid solution.

The noble metals are precipitated from the oxalate solution by adding 10 ml of a 0.66-0.6-mol/l-Na₂S water solution and raising the pH to a value of 1.2 by means of an alkali, for example a water solution of ammonia. Agitation and heating of the solution after the raising of the pH improve precipitation. The precipitate formed can be separated, for example, by sedimentation. The pH of the solution is further raised to a value of 8.5 by means of an alkali and the precipitate formed is separated from the solution.

The rare earth elements are precipitated from the sulphuric-acid solution by adding an amount of oxalic acid that is 5-20-times greater than that of the amount of rare earth elements. The pH of the solution is raised to a value of 1.2 by means of an alkali, for example a water solution of ammonia, and the solution is agitated at room temperature for 20 minutes. The precipitate can be separated from the solution, for example by sedimentation. The above examples of processes can be scaled up to production-plant dimensions. Thus the processes described also function in production conditions, in which there are tonnes, or even tens of tonnes of ash in each batch to be processed.

On the basis of the extraction tests, the ashes contain, for example, an average of 66.7-g/tn of rubidium, the market value of which corresponds to about

840 per tonne of ash, calculated according to the latest market prices of metals. Nowadays, the demand for rare earth elements has increased considerably. The so-called light lanthanides, in which are included cerium, praseodymium, neodymium, and lanthanum are regarded as the most significant in terms of demand. Their total average contents in ashes are about 250 g/tn. By means of the method, palladium, significant amounts of iridium, gold, rubidium, and platinum are also recovered, up to 2.7; 17.8; 4.2; 83.4; and 2.7 g/tn respectively. The value of even these five elements is nearly

3000 per tonne. 

1-15. (canceled)
 16. Method for processing ash, particularly fly ash, in which method several elements are separated from the ash, characterized in that in the method both noble metals and rare earth elements are separated so that the elements are separated in two extraction processes, in the first extraction process of which noble metals are separated and in the second extraction process of which rare earth elements are separated.
 17. Method according to claim 16, characterized in that the solutions obtained in the extraction processes are precipitated in two steps, in the first step of which noble metals are precipitated and in the second step of which rare earth elements are precipitated.
 18. Method according to claim 17, characterized in that the extraction processes and steps are integrated with each other.
 19. Method according to claim 16, characterized in that in the first extraction process noble metals are dissolved using a water solution with an oxalate content.
 20. Method according to claim 16, characterized in that in the second extraction process rare earth elements are dissolved out of the undissolved ash in the first extraction process, by means of a solution which is a mixture of sulphuric and nitric acid.
 21. Method according to claim 16, characterized in that the oxalate-extraction solution obtained from the first extraction process is processed in at least two stages, in such a way that a first precipitation solution containing sulphide and ammonium chloride is first of all added to the oxalate-extraction solution, in order to separate iridium and copper, the pH of the remaining solution being raised in order to precipitate the rest of the noble metals in the second precipitation stage.
 22. Method according to claim 21, characterized in that in the first stage of the precipitation process the pH of the oxalate-extraction solution is arranged to be 1.5±0.3 and in the second stage the pH of the solution is arranged to be 8.5±0.3.
 23. Method according to claim 16, characterized in that an oxalic-acid solution is added to the extraction solution obtained from the second extraction process, in order to separate rare earth elements as a third precipitation stage.
 24. Method according to claim 23, characterized in that in the third precipitation stage the pH of the extraction solution is arranged to be 1.5±0.3.
 25. Method according to claim 23, characterized in that an oxalic-acid solution is added to the third precipitation stage.
 26. Method according to claim 16, characterized in that ash undissolved in the extraction processes is washed with water in wash stages before the following treatment.
 27. Method according to claim 26, characterized in that the wash solution formed in the washing is returned to the extraction processes after the wash stages.
 28. Method according to claim 16, characterized in that the extraction processes and/or the first, second and third precipitation stages are boosted by adjusting the temperature, adjusting the pressure, agitating the solution, treating the solution mechanically, and/or directing ultrasound to the solution.
 29. Method according to claim 22, characterized in that in the second stage of the precipitation process the pH is adjusted by means of ammonia and area the nitrogen remaining after the second stage is collected as fertilizer.
 30. Method according to claim 23, characterized in that the oxalic acid used in the rare-earth-elements precipitation stage is recycled to the first extraction process. 